Resistance welding involves positioning a workpiece between a pair of electrodes through which electrical current is delivered from a power supply and then clamping the electrodes to squeeze the workpiece therebetween. Electrical current is delivered from one electrode through the workpiece to the other electrode at the point of contact between the electrodes and the workpiece. Heat generated by the resistance encountered by the current passing through a part of the workpiece melts the contact faces of the workpiece parts, thereby melting the parts to create a weld. In some cases, after the melted joint between the parts has reached a sufficiently high temperature, electrical current flow is terminated and pressure is maintained for a prescribed time interval to unite the part. In other cases, such as spot welding, the parts need not remain clamped after current flow is terminated.
One typical type of resistance welding apparatus is shown in U.S. Pat. No. 3,074,009 in which the electrodes receive electrical current from the secondary windings of a stepdown output transformer. The primary winding of the transformer is coupled in series with a capacitor, a source of A.C. electrical power and a pair of ignitrons or similar rectifier tubes. The ignitrons are phase shifted such that they begin to conduct just before the voltage peak in each half cycle of the power. Each time an ignitron begins to conduct, it places a charge on the capacitor approximately equal to this peak. The charge remains on the capacitor until the next half cycle when the other ignitron fires. The capacitor rapidly discharges its stored electrical potential into the primary of the output transformer, thereby generating a sharp pulse or spike in the secondary current of the transformer which is delivered to the welding electrodes. Thus, current is delivered to the electrodes through the transformer secondary in a series of spikes or pulses each equal to approximately twice the value of the instantaneous line voltage. This pulsed current power supply provides instantaneous localized heating of the welding surface which cannot be achieved by a lower current of proportionately longer duration because of the dissipation of the interface surface temperature by conduction. Thus, a lower value of power is required in these pulsed current systems and the undesirable secondary heating effects are minimized.
Although welding systems of the type shown in U.S. Pat. No. 3,074,009 described above are entirely suitable for many applications, they are less than completely desirable in others, and may not be used at all in some applications involving metal workpieces which must not be overheated, e.g., aluminum, dissimilar materials, coated materials, etc. It is therefore necessary to minimize the duration for which current pulses are applied to these types of materials, and in some cases the welding cycle must not exceed one or two pulses to avoid overheating the materials.
Producing satisfactory welds with only a very few number of current pulses has not been possible in the past because of limitations inherent in the power supply systems used in the prior art welders. The primary limitation resides in the manner in which the main storage capacitor is charged from the power supply circuit. When a weld is initially commenced, the capacitor is charged by an A.C. power supply, but does not reach full, steady state voltage for one or more cycles of the current. This factor is not a problem in most applications where a plurality of pulses is required to effect the weld, however, in those cases where only one or a few pulses are required, the capacitor may not have achieved a full charge during the first few cycles of current and the resulting current pulse or pulses may be inadequate in magnitude to produce a satisfactory weld.
Accordingly, it is a primary object of the present invention to overcome each of the deficiencies of the prior art systems described above.
It is also an object of the present invention to provide a power supply for pulse welders having the capability to generate voltage pulses during the first cycle or first few cycles which is greater in magnitude than the normal line voltage used to generate the pulses during steady state conditions.
Another object of the invention is to provide a power supply of the type mentioned above which includes a precharging circuit for precharging the firing capacitor to a preselected voltage and means for automatically disconnecting the precharging circuit from the capacitor when the latter is fired.
A still further object of the invention is to provide a power supply as described above which includes means for automatically discharging the firing capacitor in the event of a power failure.
These, and further objects of the invention will be made clear or will become apparent during the course of the following description of a preferred embodiment of the present invention.